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Human immunodeficiency virus 1 (HIV-1) is a life-threatening pathogen that still lacks curative therapies or vaccines. Circadian rhythms are endogenous daily oscillations that coordinate an organism’s response to its environment and invading pathogens. Recent reports of diurnal variation in peripheral viral load in HIV-1 infected subjects highlights the need for mechanistic studies. Here, we demonstrate rhythmic HIV-1 replication in circadian-synchronised model systems and show the circadian transcription factors, BMAL1 and REV-ERB, bind conserved motifs in the HIV-1 promoter. REV-ERB competes with ROR for binding to the same regulatory motif, and we uncover a role for ROR inhibitors to perturb rhythmic HIV-1 replication and host gene expression. We demonstrate that many HIV-1 host factors are circadian regulated and likely to define rhythmic viral replication. In summary, this study increases our understanding of the mechanisms underlying the circadian regulation of HIV that can inform HIV therapy and management.

Viral replication is defined by the cellular microenvironment and one key factor is local oxygen tension, where hypoxia inducible factors (HIFs) regulate the cellular response to oxygen. Human immunodeficiency virus (HIV) infected cells within secondary lymphoid tissues exist in a low-oxygen or hypoxic environment in vivo. However, the majority of studies on HIV replication and latency are performed under laboratory conditions where HIFs are inactive. We show a role for HIF-2α in restricting HIV transcription via direct binding to the viral promoter. Hypoxia reduced tumor necrosis factor or histone deacetylase inhibitor, Romidepsin, mediated reactivation of HIV and inhibiting HIF signaling-pathways reversed this phenotype. Our data support a model where the low-oxygen environment of the lymph node may suppress HIV replication and promote latency. We identify a mechanism that may contribute to the limited efficacy of latency reversing agents in reactivating HIV and suggest new strategies to control latent HIV-1., Zhuang et al. investigate how low oxygen levels affect HIV-1 replication in cell culture models. They find that the hypoxia-induced transcription factor HIF-2α represses HIV transcription and that low oxygen reduces the reactivation of latent HIV, suggesting that the hypoxic environment in the lymph node may influence HIV replication and latency.

Chromatin replication involves duplicating DNA while maintaining epigenetic information. These processes are critical for genome stability and for preserving cell-type identity. Here we describe a simple experimental approach that allows chromatin to be captured and its content analysed after in vivo replication and labeling of DNA by cellular DNA polymerases. We show that this technique is highly specific and that proteins bound to the replicated DNA can be analyzed by both immunological techniques and large scale mass spectrometry. As proof of concept we have used this novel procedure to begin investigating the relationship between chromatin protein composition and the temporal programme of DNA replication in human cells. It is expected that this technique will become a widely used tool to address how chromatin proteins assemble onto newly replicated DNA after passage of a replication fork and how chromatin maturation is coupled to DNA synthesis.

In chronic lymphocytic leukemia (CLL) the proliferation rate and resistance to drug-induced apoptosis are recognized as important factors in the outcome of treatment. In this study, we assess the activity and the mechanism of action of the prototype cell division cycle kinase 7 (Cdc7) inhibitor, PHA-767491, which inhibits the initiation of DNA replication but also has cyclin-dependent kinase 9 (Cdk9) inhibitory activity. We have studied the effects of this dual Cdc7/Cdk9 inhibitor in both quiescent CLL cells and CLL cells that have been induced to proliferate using a cellular coculture system that mimics the lymph node microenvironment. We find that this compound, originally developed as a DNA replication inhibitor, is particularly active in promoting mitochondrial dependent apoptosis in quiescent CLL cells purified from peripheral blood of patients regardless of recognized risk factors. In this setting, apoptosis is preceded by a decrease in the levels of Mcl-1 protein and transcript possibly due to inhibition of Cdk9. Following stimulation by CD154 and interleukin-4, CLL cells become highly chemoresistant, reenter into the cell cycle, reexpress Cdc7 kinase, a key molecular switch for the initiation of DNA replication, replicate their DNA, and undergo cell division. In this context, treatment with PHA-767491 abolished DNA synthesis by inhibiting Cdc7 but is less effective in triggering cell death, although Mcl-1 protein is no longer detectable. Thus, dual Cdc7/Cdk9 inhibition has the potential to target both the quiescent and actively proliferating CLL populations through two distinct mechanisms and may be a new therapeutic strategy in CLL. Mol Cancer Ther; 10(9); 1624–34. ©2011 AACR.